Coatings are applied to light transmissive or reflective members in order to impart desired optical properties to the members. For example, one or more coatings may be applied to a screen of a computer monitor in order to provide polarization or to reduce reflection. A cathode ray tube (CRT) may be surface coated with an antireflection layer having an optical thickness of approximately one-quarter of the center wavelength of light that is to be reflected. The antireflective coating may be an organic material, such as a polymer, or an inorganic material, such as a metal fluoride, metal oxide, or metal nitride. Often, the antireflective coating is a multi-layer stack.
Obtaining the desired optical properties is not isolated from other concerns. Coating a light transmissive member should not render the coated surface susceptible to chemical or mechanical damage. One approach to providing a coating having the required properties is to select a single material for forming a layer that reaches acceptable levels of antireflectivity, mechanical strength and chemical resistance. U.S. Pat. No. 5,281,893 to Matsuda et al. describes an optical film for application to a display screen of a CRT or liquid crystal display (LCD). The optical film is described as having a reduced external light reflection, a high mechanical strength and an increased resistance to damage caused by humidity. While the single-layer approach provides advantages with respect to ease of fabrication, this approach necessarily requires that tradeoffs be made among the various properties.
The more common approach is to utilize a multi-layer assembly. U.S. Pat. No. 5,170,291 to Szczyrbowski et al. describes a four-layer system having alternately high and low refractive indices in order to achieve a high antireflection effect. However, the exposed outermost layer remains susceptible to damage caused by contact with an object or a chemical. Ideally, the outermost layer is resistant to attack by common cleaners and solvents, is scratch-resistant, is not stained by water or oils, and allows easy removal of dust and other particulate matter. At the same time, the layer should not adversely affect the optical properties of the coated assembly.
U.S. Pat. No. 5,476,717 to Floch describes a multi-layer assembly for a transparent substrate. An adhesion promotion layer formed of a material selected from silanes is formed on the substrate. An antireflection coating formed from colloids of silica in a polysiloxane binder is then formed on the adhesion promoter layer. Then, a coupling agent layer of a material selected from silazanes is applied. Finally, an anti-abrasion layer of a fluorinated polymer is formed to reduce the mechanical vulnerability of the assembly. The anti-abrasion layer identified in the patent is a derivative of polytetrafluoroethylene (PTFE), which is sold under the federally registered trademark TEFLON AF and marketed by duPont de Nemours. While the anti-abrasion layer provides a number of advantages over prior art assemblies, one disadvantage is that the TEFLON AF on the silazane material must be raised to a relatively high temperature. The Floch patent identifies a final heating stage at a temperature between 110.degree. and 120.degree. C. in order to evaporate the residue solvent. Another disadvantage is that the coating may be relatively easily damaged by normal use, for example, in computer display applications.
A parameter that is used to characterize properties of a surface is contact angle, i.e., the angle formed between the plane of the surface and the tangent to a droplet on the surface. The measure of the contact angle is an indication of surface energy and may be used to characterize the wetability, oil resistance (e.g., anti-smudge) and the lubricating effects of a coating on a substrate.
What is needed is a light transmissive assembly in which tradeoffs between optical and mechanical properties are minimal and a protective layer is formed as the outermost layer in a cost efficient manner that does not jeopardize the structural integrity of previously formed layers. More specifically, what is needed is a topcoat that has a high contact angle, that maintains a high contact angle after rubbing, and that is formed at a sufficiently low temperature to not jeopardize either a substrate or an intermediate layer between the substrate and the topcoat.